This invention relates to a method of producing a polyester film using the application of static electricity, and more particularly, relates to a method of producing a biaxially stretched polyester film with regulating casting conditions of a polyester film at a high speed so as not to induce surface defects caused by entrapping air microbubbles between a cooling drum and a molten resin sheet, and regulating longitudinal stretching conditions and lateral stretching conditions. The biaxially stretched polyester film is excellent in small thickness variation, resistance to cleavage and improvement in bowing phenomenon, and is particularly suitable for the support of a photographic film.
In a conventional method of producing a cast film by cooling rapidly to solidify a thermoplastic resin containing polyester reins, the application of static electricity is employed for adhering a sheet-shaped molten resin discharged from a die to a cooling drum (Japanese Patent KOKOKU 37-6142).
Besides, it is also disclosed in a reference xe2x80x9cKobunshi Jikken Gaku 5, Polycondensation and Polyadditionxe2x80x9d, Kyoritsu Shuppan, 1980 that a metal catalyst is necessary for the synthesis of a polyester, such as polyethylene terephthalate.
Incidentally, when film forming speed of a cast film is raised in the above application of static electricity system, air entrapping problem occurs between a molten resin sheet and a rotating cooling drum exceeding a critical speed. As a result, indentations are formed as traces of air-entrapped on the surface of cooled sheet.
A considerable solving means for raising the critical speed, at which air entrapping occurs, is to increase the content of metal catalyst. That is, by increasing the content of metal catalyst, the specific resistance of the molten resin sheet is lowered, and thereby, adhesion of the sheet to a cooling drum is improved upon applying static electricity to inhibit entrapping of air. However, the increase of metal catalyst content induces problem of tone change caused by thermal degradation and the generation of metallic agglomerates in the molten resin.
Therefore, cast film forming is carried out by determining allowable surface conditions for the use of each sheet previously, preparing an optimum recipe of resin composition, and casting the sheet at a speed lower than the critical speed. Particularly, in the field of requiring severe allowable level of foreign matters, such as photographic film, there is a limitation of metal catalyst content by itself, and various proposals have been made for film forming with harmonizing film forming speed with tone change and the generation of metallic agglomerates.
For example, Japanese Patent KOKAI 2-9688 discloses a method of forming a uniform polyethylene terephthalate film by controlling the specific resistance of the polyester in a melted state to 500,000 to 500,000,000 xcexa9xc2x7cm. However, according to this method, coloring occurred caused by thermal degradation in a small specific resistance region, i.e. when metal catalyst content is great, and air entrapping cannot be inhibited at a high speed in a large specific resistance region. Accordingly, a high speed film forming of a polyester film cannot be achieved for a photographic film.
Japanese Patent KOKAI 8-291216 discloses a method of inhibiting air entrapping by restricting the specific resistance of the polyester in a melted state to 1,000,000 to 10,000,000 xcexa9xc2x7cm narrower than the above method. This method is preferable for high speed film forming, but is insufficient in tone change problem and metallic agglomerates problem for a photographic film.
Japanese Patent KOKAI 61-219622 discloses a method of forming a thermoplastic polymer sheet which comprises extruding the thermoplastic polymer in sheet on the surface of a rotating cooling drum on which a liquid membrane layer has been applied to solidify (called water application system), wherein locating the die in front of the vertical plane containing the rotating shaft of the cooling drum, and tilting the discharge angle of the molten thermoplastic polymer from the die against the vertical plane to shorten the traveling distance of the extruded sheet from the die to the surface of the cooling drum. Japanese Patent KOKAI 61-237619 discloses a modification of the above method by providing an adjusting plate for sheet edge portion on both side ends of the discharge port, and a molten resin sheet immediately after discharging travels in a state that both sides of the sheet are in contact with the adjusting plate. However, the above film forming methods according to the water application system are difficult of setting water application conditions so that traces of water drops do not remain on the surface of sheet, and a precision controller is necessary for the films requiring severed allowable level of surface defects, such as a support for a photographic film. Moreover, when the discharge angle of a die is tilted as disclosed in Japanese Patent KOKAI 61-219622, 61-237619, it is difficult to place a heat bolt for adjusting lip clearance, and a working space for rough adjustment by hand must be set higher than the cooling drum.
As mentioned heretofore, by the prior art, a polyester film without problems of tone change, metallic agglomerates and the like and useful for a photographic film support cannot be produced stably at a high speed without surface defects, such as air entrapping.
Incidentally, films useful for a support for a photographic film are required to be resistant to cleavage as well as small thickness variation. There is a prior art to decrease thickness variation by regulating longitudinal stretching conditions, such as regulating stretching distance or force, multistep longitudinal stretching, regulating film crystallinity immediately before longitudinal stretching. However, some polyesters, such as polyethylene-2,6-naphthalene dicarboxylate, do not have a one to one relationship between stress and elongation, that is, there are two or more elongation types against an stress. As a result, uneven elongation tends to occur in the width direction and the traveling direction during stretching a polyester film.
This problem can be avoided by stretching the film until the region of one to one relationship between stress and elongation. For example, in the case of polyethylene-2,6-naphthalene dicarboxylate, a stretched film without uneven thickness, i.e. having a small thickness variation, can be produced by stretching at a longitudinal draw ratio of about 5 times. However, the stretched film stretched by a longitudinal draw ratio of about 5 times is liable to be cloven, and accordingly, it is unsuitable for the support of a photographic film. When a film having resistance to cleavage required as the support for a photographic film is produced, the longitudinal draw ratio must be less than 4 times. The films stretched under conventional conditions in this region have an uneven thickness.
That is, Japanese Patent KOKAI 60-178025 discloses a means for restricting uneven Young""s modulus in the width direction and uneven thickness by setting the longitudinal stretching distance 30 to 800 mm and the stretching force 0.2 to 10 kg/mm2. However, a polyethylene-2,6-naphthalene dicarboxylate film with a small uneven thickness cannot be obtained only by the means.
Japanese Patent KOKOKU 7-71821 discloses a means for obtaining a polyester being excellent in uniform thickness by stretching longitudinally in multiple step. However, films obtained by stretching a total longitudinal draw ratio of 4.0 times or more are insufficient in the resistance to cleavage required as the support for a photographic film, and breakage occurs during processing in the subsequent processes.
Japanese Patent KOKAI 6-210719 discloses a means for restricting uneven thickness by restricting crystallinity of a film immediately before longitudinal stretching to 0.5 to 20%, and a means for crystallization is a preliminary stretching, i.e. the longitudinal stretching is divided into two steps, and the first stretching is carried out at a draw ratio of 1.2 to 2.5 times. However, even adjusting the crystallinity of a film to 0.5 to 25%, in the case of a total longitudinal draw ratio of less than 4.0 times, stretched films with a small uneven thickness cannot be obtained by the means.
As explained above, by the prior art, both of resistance to cleavage and small uneven thickness cannot be satisfied simultaneously.
Moreover, polyester films useful for the support for a photographic film are required to have a small bowing phenomenon.
The bowing is defined and evaluated by drawing a straight line in the width direction of a longitudinally stretched film at the entrance of a tenter, and measuring the bowed degree, i.e. the distance between the top of the bowed line and the straight line connected both sides of the film (C in FIG. 6) per the width of the film (W in FIG. 6) at the exit of the tenter. The less the bowing is, the less the distribution of properties in the width direction is, and accordingly, is preferable for the support for a photographic film. It is considered that the bowing phenomenon is due to the distribution of molecular orientation in the width direction occurring caused by the thermal shrinkage at the central portion which is not fixed by a clip during the process of stretching a longitudinally stretched film laterally by a tenter.
Some trials were made for decreasing the bowing, such as by providing an intermediate cooling zone having a length of the film width or more between the stretching zone and the heat setting zone, by cooling to the glass transition point or lower than that in the intermediate cooling zone, restretching in the cooling process, and relaxing in the lateral direction in the cooling process, or the like. However, these technics are insufficient, because there is a region where the bowing phenomenon cannot be improved according to the type of resin, conditions, such as speed, or the like.
That is the method disclosed in Japanese Patent KOKAI 3-193328 was made for the purpose of improving the bowing which comprises providing an intermediate zone having a length of the film width or more between the stretching zone and the heat setting zone, and cooling the film to the glass transition temperature or less. However, according to this method, the improvement in the bowing was not exhibited in the case of polyethylene-2,6-naphthalate film.
Japanese Patent KOKAI 3-216326 and 4-142916-8 disclose some modifications of the above method, such as restretching in the cooling process, relaxing in the lateral direction in the cooling process or the like. However, these methods were also not effective for the improvement in the bowing as to polyethylene-2,6-naphthalate film.
Moreover, the above methods are difficient in the care for raising a film forming speed, such as cooling conditions, and are inferior in generality.
Japanese Patent KOKAI 3-193329 and 3-207632 disclose the modification of the above method which provides nip rolls in the intermediate zone. However, nip rolls tend to generate abrasion and lateral unevenness, and therefore, the method is not suitable for the production of a support for a photographic film.
As mentioned above, by the prior art, a film for a support for a photographic film having a small bowing phenomenon cannot be supplied stably even varying film forming speed.
As explained heretofore, although various technics for producing polyester film have been developed, it is not possible to produce a polyester film having no problem of tone change, metallic agglomerates and the like stably at a high speed without generating surface defects, such as air entrapping. Moreover, it is not possible to satisfy both of resistance to cleavage and small uneven thickness, nor possible to supply stably a polyester film having a small bowing phenomenon even varying film forming speed.
An object of the invention is to provide a method of producing a biaxially stretched polyester film, capable of forming a cast film having stable surface conditions in a region of higher casting speed than the conventional method without inducing tone change and metallic agglomerates caused by thermal degradation, and producing stably the biaxially stretched polyester film being excellent in resistance to cleavage, small thickness variation and small bowing phenomenon even varying film forming speed.
The inventors investigated eagerly in order to solve the above problems, and devised a method which has achieved the above object, wherein restricting electrostatic resistance (quantity of blended metal catalyst) of the resin to be used and setting the die in a prescribed range in the casting process of the cast polyester film, rendering the longitudinal stretching process into two step stretching under the restricted conditions of position relations, such as the gap between far infrared heater(s) and stretching roll(s) and restricted stretching conditions, such as draw ratio in each stretching process, and restricting tenter conditions, such as stretching temperature in the tenter, temperature of intermediate zones, passing time and heat setting temperature.
Thus, the present invention provides a method of producing a biaxially stretched polyester film which comprises casting a polyester cast film by extruding a molten polyester resin onto a cooling drum through a die while static electricity is applied to the extruded molten polyester resin, stretching the cast film in the longitudinal direction by stretching rolls, stretching the longitudinally stretched cast film in the lateral direction by a tenter, and heat setting the biaxially stretched cast film, wherein
said molten polyester resin having been adjusted so as to have a specific resistance of 5xc3x97106 to 3xc3x97108 xcexa9xc2x7cm,
said die being positioned so that the line connecting the center of the cooling drum and the intersection of the vertical line passing the die lip with the circumferential surface of the cooling drum has an angle of 30 to 90 degrees against the vertical line passing the center of the drum,
said stretching in the longitudinal direction comprising a first longitudinal stretching and a second longitudinal stretching, each with heating by a far infrared heater, wherein stretching rolls in each stretching are located with a gap of 3 to 40 mm, and each far infrared heater is located with a gap of 5 to 40 mm from the circumferential surface of the stretching roll,
wherein the cast film being stretched at a draw ratio of 1.05 to 1.5 times in the first longitudinal stretching, being cooled to glass transition temperature or lower, and then being stretched again so that the total draw ratio becomes 2.5 to 3.9 times in the second longitudinal stretching,
said stretching in the lateral direction being carried out at a temperature in the range from glass transition temperature to the glass transition temperature +50xc2x0 C.,
passing the laterally stretched cast film through an intermediate zone kept at a temperature from the surface temperature of the cast film at the end of the lateral stretching +20xc2x0 C. to the maximum temperature of the surface temperature of the cast film in the tenter xe2x88x9220xc2x0 C. for 3 to 30 seconds, and
said heat setting being carried out at a temperature from melting point xe2x88x9230xc2x0 C. to melting point xe2x88x925xc2x0 C.